Exam 4: Tubular Exchange Mechanics 167 - 190 Flashcards
What is the primary function of the nephron after the glomerulus?
Reabsorption of glomerular filtrate
What is suggestive of where most of the reabsorption work is done in the nephron?
Flow rate of glomerular filtrate through the nephron
What part of the nephron does the bulk of the reabsorption work?
Proximal convoluted tubule
What percentage of water reabsorption occurs in the proximal convoluted tubule?
70%
How does the kidney handle the processing of nutritionally important substances such as glucose and amino acids?
Very well - almost absorbed to complete
How does the kidney handles processeing ions and electrolytes such as sodium, chloride and bicarbonate?
Well, although variably. Can be absorbed to near completion but not as well as glucose and amino acids
How does the kidney handle processing metabolic end-products such as urea, creatine, phosphates and sulfates?
Poorly, they are usually not well recovered by kidney filtration
What is the basic method of tubular exchange for glucose (and galactose)?
Secondary active transport using a cotransporter and facilitated diffusion with GLUT2 transporter protein
Does glucose transport, as seen in tubular exchange, require energy? If so, why and in what form?
Yes. It requires ATP to drive the Na+/K+ exchange pump that allows to cotransport of Na+ and glucose from the PCT lumen
Glucose transport in tubular exchange requires energy and uses secondary active transport. What other molecules are transported in essentially the same way?
- other simple sugars
- carbohydrates (after being broken down to simple sugars)
- amino acids
- proteins (after being broken down to amino acids)
Total glucose absorption is a function of what?
The number of transporters which is a function of surface area
I.e. # of nephrons, # of microvilli, length and convolution of tubules will all affect surface ares
Is glucose absorption in the kidneys efficient?
Yes, normally it is so efficient that it is absorbed to completion
What is the definition of tubular transport maximum for glucose?
The total glucose that can be absorbed and it represent an estimate of the number of functioning nephrons
What is the average tubular transport maximum for glucose?
~ 350 mg/min (±50mg/min)
NOTE: this means that the tubule can absorb 350mg of glucose per minute
What is the definition of the renal threshold of glucose?
The point at which tubular transport cannot keep up with glomerular filtration rate and glucose might appear in the urine
What is the theoretical value of the renal threshold for glucose?
~ 300mg / 100 ml of plasma
Although actual threshold (when glucose appears in the urine) is 180-200 mg/dL
NOTE the formula used: Renal threshold = Tubular transport maximum for glucose (~300-400mg/ml) / glomerular filtration rate (125 ml/min
Do the kidneys normally regulate blood glucose?
No
What is glucosuria?
Glucose in the urine
What is the basic method of tubular exchange for simple sugars other than glucose (and galactose)?
Still secondary active transport, however it utilizes specific transporters to each kind of sugar
What is the basic method of tubular exchange for carbohydrates?
The same as simple sugars (secondary active transport with specific transporters) but only after being broken down by surface amylase in the PCT
What is the basic method of tubular exchange for amino acids?
Secondary active (ATP!) transport with cotransporter and transport protein specific to each amino acid
What is the basic method of tubular exchange for peptides?
The same as amino acid (secondary active transport with specific transporters) but only after being broken down by surface peptidases in the PCT
What is the average dietary intake of sodium?
~ 8-15 grams/day
The amount recovered in the nephron directly matches the amount of salt (NaCl) intake until there is excess and then excretion rises
What is the basic method of tubular exchange for sodium (and chloride)?
primary and secondary active transport
- linked to cotransport of sugars and amino acids
- chloride generally follows sodium to balance charges
What percentage of sodium resorption occurs in the PCT?
~65%
What percentage of sodium resorption occurs in the ascending limb of the loop of Henle?
~25%
What percentage of sodium resorption occurs in the DCT?
~5%
What percentage of sodium resorption occurs in the collecting duct?
~5%
Resorption of sodium can be controlled indirectly by:
Body osmolality (partly under control of aldosterone)
What would increased aldosterone do to sodium resorption in the nephrons?
It would increase sodium resorption
- the hormone operates on the distal tubule by altering the number of Na+/K+ exchange pumps and their activity
What would decreased aldosterone do to sodium resorption in the nephrons?
It would decreased sodium resorption
- the hormone operates on the distal tubule by altering the number of Na+/K+ exchange pumps and their activity
Regulation of sodium by aldosterone is considered a:
Distal nephron function, because the hormone operates on the distal tubule by altering the number of Na+/K+ exchange pumps and their activity
How does increased sodium concentration in filtrate passing through the distal tubule (and macula densa cells) affect aldosterone secretion?
Decrease aldosterone secretion
↑ Na+ stimulates neighboring JG cells
(Which communicate with macula densa cells) to ↓ secretion of renin (and angiotensin)→ ↓ aldosterone secretion → ↓ Na+ resorption → more excretion of Na+
How does decreased sodium concentration in filtrate passing through the distal tubule (and macula densa cells) affect aldosterone secretion?
Increases aldosterone secretion
↓ Na+ stimulates neighboring JG cells (which communicate with macula densa cells) to ↑ secretion of renin (and angiotensin) → ↑ aldosterone secretion→ ↑ Na+ resorption → less excretion of Na+
Changing Na+ intake will alter water retention due to:
Osmosis
Are the kidneys the prime regulator of the body sodium (and therefore chloride) content? Why or why not?
Yes, because the vast majority of NaCl that is lost by the body is by way of urine
Normally, where is potassium found in higher concentrations?
Inside the cells…98% of K+ is intracellular
What percentage of resorption of potassium is done in the proximal tubules?
~70%
What percentage of resorption of potassium is done in the loops of Henle?
~20%
What percentage of resorption of potassium is done in the distal tubules and upper collection duct?
~10%
What is the mechanism of resorption of potassium in the PCT?
Passively with NaCl and water
What is the primary mechanism of potassium resorption in the distal tubules?
Nephric tubule cell sub types function to adapt to the bodies needs: α-intercalated cells resorb and principle cells secrete
What nephric cell sub type functions in response to ↓ K+ intake?
α-intercalated cells which will lead to potassium resorption into blood and less excretion
What nephric cell sub type functions in response to ↑ K+ intake?
Principle cells which lead to potassium secretion from blood and more excretion from body
NOTE: this is the main action and ensures the potentially dangerous cation, K+, does not rise too high
How is the main action of potassium filtration through principle cells in the distal tubules controlled?
By way of aldosterone
What effect does increased aldosterone have on potassium?
It decreases K+ concentrations in the blood by promoting secretion in the distal tubule via principle cells
What effect does decreased aldosterone have on potassium?
It increases K+ concentrations in the blood by promoting resorption/minimizing secretion in the distal tubule via α-intercalated cells
What is kaliuresis?
Potassium in the urine
Are the kidneys the primary regulator of body potassium content? Why or why not?
Yes, because the vast bulk of potassium is lost in the urine under normal conditions
What is the solvent of the body?
Water
What is the average intake of water?
~ 2.5 liters/day
Are the kidneys the primary regulator of body water content? Why or why not?
Yes, because though water can be lost from other sources, the bulk of it is lost as urine
Which is more difficult for the kidneys - disposing of water or retaining water?
Retaining water
What percentage of water resorption is done at the proximal tubules?
~70%
What percentage of water resorption is done in the loops of Henle?
~15%
What percentage of water resorption is done in the distal tubules?
~5%
What percentage of water resorption occurs in the collecting duct?
~10%
The primary job of the nephron is:
to adjust body water levels by producing either a dilute or concentrated urine!
excess body water turns into:
a dilute urine (↑ urine volume)
lack of body water turns into:
a concentrated urine (↓ volume)
What is osmolality (aka osmolarity)?
is a measure of the number of osmotically active particles in a solution and therefore a measure of the tendency of a solution to induce osmosis (the diffusion of water)
On average, what is the normal body fluid osmolarity?
- body/blood osmolality of ~300 mOsm
NOTE: The body is capable of adjusting urine to be as dilute as ~50 mOsm/l to as high as ~1200 mOsm/l
How is the bulk of water resorbed in the PCT?
by simply following the “osmotic gradient” caused by the resorption of NaCl
How is water resorbed in the DCT?
By following Na+ (promoted by aldosterone)
Are collecting ducts permeable to water?
No, they are relatively impermeable
Without hormonal influence, the relative impermeability of the collecting ducts (where ~10% of water can be resorbed) will lead to what?
Renal dilution of urine
With hormonal influence from ADH, the relative impermeability of the collecting ducts can be overcome and lead to what?
Renal concentration of urine because ADH makes the ducts more permeable to water, driving water out and back into peritubular capillaries
What is a counter current system?
Any pipe that doubles-back on itself allowing it to communicate with itself and magnify its effects as seen in osmolality magnification of the loops of Henle
Why are the thick ascending segments of the loop of Henle said to be “active”?
Because they actively pump NaCl into the interstitium while the descending thin segments attempt to equilibrate with the interstitium
What is the effect of the thick ascending limb of the loop of Henle actively pumping NaCl?
Hypertonicity of interstitium
What are diuretics?
Agents that cause water lose (aka urine dilution)
Describe the permeability of the descending limb of Henle to NaCl and water
impermeable to NaCl but permeable to water
How do the kidneys regulate the concentration of H+?
by ↑ or ↓ [HCO3-]
Remember the reversible bicarbonate reaction?
bicarbonate is reabsorbed primarily in ____ but finished in _______.
- PCT
- distal nephron
What determines the degree of hypertonicity of medullary interstitium?
The length and number of loops of Henle
The vasculature of the nephron allow for what to happen to water and NaCl?
Water effectively by-passes the medulla and NaCl is entrapped in there.
Water out and NaCl in as blood descends into medullary center.
Water in and NaCl out as blood ascended from medullary center
What is the function of ascending vasa recta?
To capture resorbed H2O from the collecting ducts
Where is ADH produced?
Hypothalamus which project axons to the posterior pituitary where the hormone is released
What is the fate of ADH once it has been released from the posterior pituitary?
it lands on receptors (V2) of the collecting ducts and causes the insertion of reserve pools of cytoplasmic aquaporin-2 proteins that will greatly increase the permeability of the collecting duct cells
The final acidity of urine is complete in:
The distal tubules
What does it mean that H+ is titrated against filtered HCO3-
H+ combines with HCO3-, and H+ is ultimately lost as H2O in urine
What is acidosis and what causes it in the urine?
Decreased pH (acidic) urine) Increased H+ levels exceed the available HCO3-
What is alkalosis and what causes it in the urine?
Increased pH (alkaline) urine because the HCO3- levels exceed the available H+ secretion
H+ in the distal nephron must be buffered to protect the body on the way out. What 2 chemical systems are used to do this?
◦ Phosphate buffer: HPO4-2 + H+ → H2PO4- ◦ Ammonium buffer: NH3 + H+ → NH4+
Why can ammonia sometimes be smelled in urine?
Ammonium ion is converted back to ammonia when exposed to air
How is urea filtered by the nephrons?
It uses aquaporin channel to diffuse through the collecting ducts
NOTE: it diffuses much slower than water and therefore tends to pass right out with the urine
Is calcium primarily intra or extracellular?
Extracellular
Is phosphate primarily intra or extracellular?
Intracellular
The kidneys have a natural tendency to do what with calcium and phosphate?
keep (reabsorb) PO4-2 and excrete Ca+2 such that serum calcium increases
The resorption of phosphate and the excretion of calcium can be modified by what?
parathormone (PTH)
